Evolution Doesn’t Explain Cowbirds and Other Brood Parasites

The first type of brood parasitism occurs within species. This is particularly common within birds with more than 50 species known to perform the practice1 but exists in other groups as well. Despite being more common in birds, only roughly 2% of birds use this breeding strategy.2 In many cases, the percentage of affected nests is very low.3^,4 Most of the time the number of chicks unrelated to either parent in a nest is around 10% or less.5^,6^,7 However, particularly in colonial breeders, this number jumps significantly.

The reasons for birds laying eggs in the nests of other members of the same species are unclear and may vary by species.

The reasons for birds laying eggs in the nests of other members of the same species are unclear and may vary by species. In one species, it was speculated that the parasites simply did not have nests and needed a place to lay their eggs.8 In another study, it was suggested that females that mated but could not find a nest site would use other nests as a place to lay their fertilized eggs.9 In one biological model, researchers proposed that laying at least one egg in another nest can increase the number of eggs laid in a bird’s own nest.10 However, as the authors themselves admit, there is no field evidence to support this model. Two other explanations, the quasi-parasitism hypothesis and the kin selection hypothesis, have been proposed. In the first instance, the female mates with the male at the nest, thus making the egg a half-sibling to the eggs already in the nest. In the second, the egg is laid in the nest of a close relative. However, a study in moorhens rejected both hypotheses. Parasitized males did not fertilize the eggs of the parasitic females, and close relatives were not preferred over other eggs.11

Interestingly, at least one species will avoid laying eggs in a nest where there is evidence of external parasites, such as ticks.12 Obviously, such a behavior is beneficial to the future offspring, as parasites have a negative effect on their growth and development. This, however, raises the question of why parasitized members of the same species simply do not reject the foreign egg, or later, the chick.

In some cases, eggs are rejected, either by abandoning the nest or ejecting the foreign egg. In one colonial species, any egg added to the nest more than four days after the first egg is laid is rejected.13 Clearly, though, egg rejection is not absolute, or brood parasitism would be a worthless behavior. In one species, only 34% of nests parasitized had an egg removed.14 Even if the parasitic egg is laid early, it is only accepted less than 40% of the time in some species.15 Part of the reason for the success rate being so high is eggs of the parasite are often all but indistinguishable in size from the host eggs.16 While they are the same species, eggs will vary in size, much like human babies vary in size at birth, so it would not be expected that such a tight match would exist, unless it was intentional.

However, the fact that not all eggs are rejected may not be entirely due to successful deception on the part of the parasite. Some hosts simply might not care. There could be several reasons why hosts might knowingly accept an egg from a parasite. One article lists several including the following:

1. Risk of error
2. Benefit to host
3. Prevent future parasitism
4. Male acceptance17

The first one on the list is self-evident. Most animals either cannot count or lack the memory to know how many eggs were in the nest the last time they visited it. If an egg is ejected or a nest is abandoned, they risk destroying their own eggs. Eggs cost a lot in terms of energy, and simply abandoning those eggs because one might not be the host’s could be worse than rearing an unrelated chick.

The second one is slightly less obvious. Rearing the parasite’s chick shows no obvious benefit to the host. However, if the predation risk is high, then spreading that risk onto an extra chick might be worthwhile.

The logic of the third and fourth reasons are slightly more complicated. In number three, it is assumed that once a nest has been parasitized, it will not be parasitized again, unless the egg is rejected. However, there can be more than one parasitic female in an area, so there is no guarantee that accepting one egg will ensure other parasites leave the nest alone. This would particularly be true in a colonial breeding species. In the fourth instance, it is assumed that males guarding the nest will allow parasitism if they get the chance to mate with the parasitic females. There is little evidence for this, especially since female birds are often the ones spending the most time at the nest.

Interspecific Brood Parasitism: Fish

While birds are the most commonly thought of brood parasites, it occurs in fish and insects as well. The most famous of these is the cuckoo catfish (Synodontis multipunctatus). Several members of the Synodontis genus are kept in aquariums, including S. multipunctatus under the label “upside down catfish.” S. multipunctatus uses mouthbrooding cichlid species as their hosts. The female cichlid lays her eggs as she swims, then scoops them up in her mouth. The cuckoo catfish exploits this process by following along and laying its own fertilized eggs among the cichlid eggs so that both end up in the female’s mouth.18 Both young then develop in the mother’s mouth, though the recently hatched catfish often eat all their host’s young (but not each other) during development.19 They are not selective about their hosts, being willing to use most mouthbrooding species.20

However, catfish are hardly the only species to engage in brood parasitism in a broad sense. Multiple species of damselfish young are reared and protected by members of other genera.21 Other species lay their eggs in the nest of other fish, allowing them to be protected until they hatch.22 Others go a step further, replacing the eggs of the nesting fish and allowing their young to be raised to independence by their host.23

Particularly in the cuckoo example, the brood parasitic behavior must be learned, rather than instinctual. Other members of the genus are substrate spawners and do not use cichlids or any other fish as hosts for their eggs. Further development timing in S. multipunctatus is similar to related species.24 In favor of this is the fact that cuckoo catfish get better at being parasites as they get older and more experienced.25 Other species can also learn to use new hosts when given the opportunity.26 As with intraspecific brood parasitism mentioned before, the foreign eggs are often rejected. Cuckoo catfish do not mimic host eggs, and in situations where their hosts are familiar with them, many of their eggs are rejected.27 Therefore, it is important for the parasite to be good at their job so that their eggs will get to develop in the host.

Interspecific Brood Parasitism: Birds

Brood parasitism in birds has been well-known for a long time and much ink has been spilled documenting and attempting to explain its prevalence.

Brood parasitism in birds has been well-known for a long time and much ink has been spilled documenting and attempting to explain its prevalence. According to the evolutionists, brood parasitism (where a different species is the host) evolved separately at least seven times,28 though at least one prior study reduced it to six.29 There are many similarities between intra- and interspecific brood parasitism in birds but also some key differences in behavior.

One key difference is that at least some of the parasites use multiple host species, often from different bird families. Cowbirds, among the most common species of brood parasites, are known to have over 100 potential hosts.30 However, some brood parasites use a single host. In fact, indigobirds are thought to have speciated based on their adherence to specific hosts.31 In at least one species of parasite, hosts are selected based on having eggs with similar color patterns.32 This choice may be because at least some of their hosts choose which eggs to reject or retain based on color patterns.33

It is both obvious and documented that brood parasitism is bad for the host.34 Thus, as noted above, the host will attempt to avoid fostering other species’ chicks. Some of their hosts have the ability to recognize foreign eggs whether the parasite is present or not and reject them.35 Others will abandon the nest after hatching if the only surviving chick is a parasite.36 Still, others simply accept the parasitism, gambling that the parasite’s eggs either will fail to hatch or will not cost the lives of their chicks.37 If they get this bet wrong, however, there is a good chance their chicks will be outcompeted by the parasitic chick that often hatches earlier and begs for food louder than their own chick.38 Some species actively recognize the parasitic parents and attack them when they are found in the area.39 Sometimes multiple members of the same species will attack the parasites in a behavior called mobbing.40

Incredibly, one species of bird responds to potential brood parasites by teaching its young, still in the egg, a special begging call. This call identifies the newly hatched chicks as theirs, rather than the parasites. The parents then preferentially respond to these begging calls when divvying out food at the nest.41

However, in some species, the lack of defense against brood parasites is referred to as “evolutionary lag,” meaning that the host species simply has not had enough evolutionary time to realize it is being parasitized.42 This is simply incoherent with history. As pointed out in a book-length study of the English bird Dunnock, they have been the host of a parasite for at least 600 years, a minimum of 100 generations!43 Given that new species of birds have developed in three generations among Darwin’s finches44 and a population of songbirds adapted to urban environments by increasing their boldness in less than 30 years,45 the idea that it would take more than 600 years to adapt to a parasite is ludicrous.

Where Did It Come From?

This argument also assumes that intraspecific brood parasitism is a stepping stone to interspecific brood parasitism, something the evolutionists’ own phylogenetic studies have largely rejected.

Several explanations have been proposed to explain the origin of brood parasitism. One hypothesis was that females might react to the destruction of their nest by laying in another bird’s nest. This, however, has been experimentally tested in blackbirds and falsified.46 This is a particularly applicable test because cowbirds, which are obligate brood parasites, are members of the same subfamily and thus close relatives. If such a behavior existed, it would be expected that red-winged blackbirds would express it. Two other species have been tested and found to willingly deposit their eggs in other nests if their own is destroyed.47^,48 These studies do nothing to further the hypothesis. First, both species tested are colonial breeders, making it very easy to deposit eggs in another nest of the same species. Second, both species exhibit intraspecific brood parasitism as normal behavior. Thus, the birds parasitizing other nests post nest destruction tells us nothing about how the behavior arose, only about how the birds respond after it did so. This argument also assumes that intraspecific brood parasitism is a stepping stone to interspecific brood parasitism, something the evolutionists’ own phylogenetic studies have largely rejected.49

A second explanation involves something called sexual imprinting. Basically, many birds learn their songs from listening to their parents and select their mates based on song. If a bird is raised in the nest of a different species, often it will learn their song and try to mate with members of that species on reaching maturity. At least one case of a hybrid brood parasite exists that follows exactly the above pattern. An egg was laid in the wrong nest, imprinted accordingly, and then mated with a member of the species that parasitized its foster species, creating a hybrid.50 Such imprinting has been proposed as a mechanism to attract brood parasites to their hosts’ nests so they can lay eggs.51

There are huge problems with this idea too. The host species must, by definition, have a nest. Why was nest building lost in the parasite? Imprinting did not do that. Further, if the imprinting hypothesis were true, flocks of hybrids between cowbirds and their hosts would be expected. The family containing cowbirds is relatively large, yet there is no proliferation of hybrids between cowbirds and the red-winged blackbird, the western meadowlark, or the eastern meadowlark, all of which are host species from the same family.52 Thus, either imprinting is too weak to induce regular mating, or cowbirds mate based on other signals. In either case, imprinting cannot explain the origin of brood parasitism, at least in cowbirds. Also, as noted above, cowbirds use multiple hosts within the same species, with some females even laying eggs in multiple species’ nests in a single breeding season.53

Imprinting may, however, explain why certain species of brood parasites, like indigobirds, are host specific. When both males and females indigobirds are raised under the song of a specific unrelated host (mostly finches), they are likely to mate with birds that have the same song, thus keeping the indigobird lineage tightly connected to the host species. This does not explain the origin of brood parasitism. Put candidly by the late great creationist ornithologist Jon Ahlquist, “Evolutionists have no idea of the origin of brood parasitism. Their speculations and ‘just so stories’ simply do not make a plausible account.”54

Brood Parasitism and the Fall

From a creationist perspective, brood parasitism is a unique study. It almost certainly did not exist, at least in its current form, before the fall. It is difficult to think of cowbird chicks pushing their nestlings out of the nest as very good. Further, we know there was no animal death prior to the fall, something that occurs in many instances of brood parasitism either of the parasitic egg/chick or of its host. Post-fall is another matter, however. We know that various attack and defense structures and behaviors developed after the fall. Some animals today are close to obligate carnivores, a state that did not exist in the pre-fall perfect world. Like carnivory, brood parasitism must have arisen in the aftermath of the fall.

The exact route to brood parasitism is unclear.

The exact route to brood parasitism is unclear. It could have involved a mistaken egg deposition, followed by imprinting, though this seems unlikely, if for no other reason than the imprinted chick would have needed a mate. In the case of the cowbird, other members of the kind do not share its lifestyle, making it likely that the behavior arose post-flood. The responses to brood parasitism, particularly egg rejection and teaching begging calls to the chicks, could have been there from the beginning, though only implemented after the fall. Beyond that, far more research is needed.